Why Are Mute Swans Called Mute Swans?

When you see swans on the ground, or on the river, there is no obvious reason why mute swans are called mute swans.

The mute swan is one of the largest flying birds.

They seem anything but silent, honking away to one another. It is when they are in flight the differences are apparent. All species of swan emit sound and communicate in the air, except for the mute swan.